Video random access memories (also known as dual port dynamic RAM's and video random access memories, or VRAM's) arc memory devices having both a port enabling random access to the memory device and a serial port for providing access for transferring data to and from the memory device. The two kinds of ports are necessary because, while memory access by a central processing unit (hereinafter CPU) is performed randomly, memory access in case of reading data to be displayed on a display device (e.g., cathode ray tube) is performed serially. The video random access memory or dual port dynamic RAM is used in applications were the memory device must perform both of these functions.
In order to increase the speed of the video random access memory, the time required for serial data transfer operations must be minimized. Serial data transfer operations of the video random access memory include both serial read transfer operations and serial write transfer operations. In the serial read transfer operation, data stored in the random access memory is transferred to a serial access memory (also referred to as a SAM); the data is then read out of the serial access memory through the serial access port. In the serial write transfer operation, data is first written into the serial access memory and thereafter transferred from the serial access memory to the random access memory.
The serial write transfer operation is of particular importance and especially the processing speed of the serial write transfer operation. Conventional serial write transfer operations are slow, restricting the overall speed at which data to be stored in the video RAM can be processed in, for instance, image processing. In applications using special integrated memory devices such as video RAM's, as well as in application using related devices such as field memories and graphics memories, the time for the serial write transfer operation should be minimized as much as possible by optimizing both design and operation of the devices.